Dynamic consolidation of powders using a pulsed energy source

Abstract

A gas operated part forming die apparatus has compact high tonnage presses which are operated by high pressure gas generated within chambers and controlled to operate high pressure pistons and dies for compressing particulate material into dense formed parts. Combustion chambers are filled with pressurized mixtures of combustible gases and diluents. Elongated chambers have insulating walls and spaced electrodes. Some contain liquid or particulate ablatable materials or ablatable liners. Others extend fuzes between the electrodes and are filled with pressurized gases. Gas is removed from the particulate material. Die cavities may be precompressed during filing of chambers with pressurized gas. Igniting the combustible gases or creating arcs between the electrodes produces rapidly expanding high pressure resultant gases for driving pistons and movable dies and rapidly compressing die cavities. Pressures in the chambers are contained, or pistons are restrained until releasing and driving the pistons. Large area pistons drive smaller movable dies.

Description

[0001]This application is a continuation of application Ser. No. 09 / 902,784 filed Jul. 12, 2001, now U.S. Pat. No. 6,767,505, which claims the benefit of U.S. Provisional Application No. 60 / 217,728 filed Jul. 12, 2000.[0002]This invention was made with Government support under Contracts DASG60-97-M-0115 and DASG60-99-C-0024 awarded by the Ballistic Missile Defense Organization. The Government has certain rights in this invention.BACKGROUND OF THE INVENTION[0003]1. Field of the Invention[0004]This invention relates in general to the field of powder metallurgy and in particular to an improved and less expensive method and apparatus for generating controllable pressure pulses (both in the shock regime and in a rapid, but shock-free, regime in the same device) for the purpose of consolidating (compacting) powders to a contiguous rigid form, primarily for the purpose of producing material samples and manufactured parts. In this field, the emphasis is generally on producing higher quality...

AI Technical Summary

Benefits of technology

[0071]Still another object of this invention is to provide a method and apparatus for consolidating nanophase or nanosized powders (be they magnetic or non-magnetic) at substantially room temperature or at temperatures of a few hundred degrees Celsius, and to perform the consolidation during a time so short that grain growth is reduced or eliminated altogether.

[0156]This technology can be developed as a cost effective, ultrafast consolidation technique to produce P / M components on an industrial scale. It is expected that the invention can provide a new processing technique that will provide safe, reliable, lightweight, and less expensive components for commercial, aerospace, and military markets. This technology will be transferable to consolidation of metallic, intermetallic, ceramic, and composite metal-ceramic materials for magnetic, electronic, and structural applications in many industries.

Problems solved by technology

However, the high cost of powder metallurgy technology must compete with other low cost manufacturing methods.

In practice, the available materials must compromise some of these properties in favor of others.

For example, the eddy current losses arise because soft magnetic materials generally operate in alternating fields.

The losses arise primarily because of difficulties in reversing the magnetization state of the material.

The eddy current losses also increase as the size of the magnetic regions increases and as the resistivity decreases.

Practically, the consolidation of nanocrystalline and amorphous particulate into useful engineering devices provides many unique challenges.

However, exposure to elevated temperatures significantly degrades the hard-won microstructural features through crystallization and grain growth.

Using binders, however, dilutes the amount of magnetic material in the final part to, at most, 70 percent by volume, resulting in a lower saturation magnetization by volume in the final engineering component.

Most dynamic consolidation techniques, however, are not amenable for large-scale production techniques.

This multiple domain state leads to relatively easy demagnetization and poor hard magnetic properties.

Because of this requirement of a fine grain size, non-equilibrium processing techniques are required.

However, during the relatively prolonged times at elevated temperatures, grain growth occurs, resulting in deleterious effects on the magnetic properties.

Die upsetting results in crystallographic alignment through preferred grain growth.

However, the additional exposure to elevated temperatures during die upsetting further degrades the microstructure.

These dynamic consolidation processes result in extremely short exposure to elevated temperatures, allowing the fine microstructures generated during melt spinning to be retained.

However, scale-up of the previously mentioned dynamic consolidation techniques provide unique challenges.

Intermetallic compound powders made by rapid solidification processing (RSP) are brittle and hard, making it difficult to consolidate these powders by conventional techniques.

The high temperature exposures for long times, involved in the conventional techniques, causes excessive grain growth and phase transformation of the initially RSP microstructure.

Also, room-temperature brittleness is a problem with conventional intermetallics.

The challenge, then, is to develop powder consolidation techniques that do not adversely affect any enhancements due to RSP.

These are all very large, noisy, complex, and expensive systems.

The limitations of conventional die pressing include limited green density, limited green strength, green density gradients, need for binders and binder removal (a slow and expensive process), and shrinkage of parts during sintering.

While higher green densities with less density gradients are reached at a given pressure when compared to die pressing, most of the other limitations of die pressing exist in CIP.

The long time exposures at high temperatures lead to grain growth and loss of the initial fine and rapidly solidified microstructures.

A disadvantage with reactive sintering is the difficulties associated with HIP, which often accompanies or succeeds reactive processing, such as the need for ductile canning materials that do not react with the powders and the need to seal cans carefully.

However, the principal disadvantages are the difficulty of complete binder removal and the inability to produce continuous fiber-reinforced composites.

Such temperatures are difficult to achieve by other powder-vehicle compaction methods that require transfer of either a preheated preform or the heated medium into the compaction vessel.

As the shock wave passes through the powder, it gives up most of its energy at particle boundaries, possibly because of interparticle friction.

The same composite when hot pressed resulted in a thick (more than 1 micrometer) fiber / matrix reaction zone that is deleterious to the fracture properties of the composite.

For example, some powders are so brittle that conventional isostatic pressing may cause excessive particle fractures.Dynamic consolidation has the potential for fabricating net-shape parts.Although most samples produced to date have been small (centimeter size), the process can, in principle, be scaled up to produce large (meter size) compacts.

Cracking is another problem that must be solved.

Also, the rarity of kinetic energy storage machines and the small sizes of samples produced to date make it difficult to predict whether kinetic energy discharge is a commercially viable way to consolidate powders.

One limitation of DMC is the general need for post sintering, under a reducing atmosphere such as hydrogen, or HIP to complete grain bonding.

Note that the quoted peak pressure of 1400 MPa apparently leads to damage to the coil, and a consequently limited lifetime.

Making survivable coils which can operate repetitively at pressures above 5-6 kbar is difficult.

All known techniques, both static and dynamic, are in general not cost effective compared to conventional wrought processing techniques and none of the dynamic techniques have generated any significant commercial activity.

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